1. Field of the Invention
The present invention relates to standardized mechanical interface (SMIF) systems for reducing particle contamination, and more particularly to transportable containers which are sealable to prevent influence of external factors on the contents of the containers.
2. Description of the Related Art
A standardized mechanical interface (SMIF) system has been proposed by the Hewlett-Packard Company as disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of the SMIF system is to reduce particle fluxes onto wafers. This end is accomplished, in part, by mechanically ensuring that during transportation and storage the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment. The SMIF concept is based on the realization that a small volume of controlled (with respect to motion, air flow direction and external contaminants), particle-free air provides a clean environment for wafers. Further details of one proposed system are described in the article entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.1 micrometers to above 200 micrometers. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are 1 micrometer and under. Unwanted contamination particles which have geometries measuring greater than 0.05 micrometers substantially interfere with 1 micrometer geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.5 micrometers and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafer cassettes; (2) canopies placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and canopies (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
Wafers are stored and transported in pods, and are transferred from a pod to a piece of processing equipment in the following manner. First, a pod is placed at the interface port on top of the canopy. Each pod includes a box and a box door designed to mate with doors on the interface ports of the processing equipment canopies. Then, latches release the box door and the canopy port door simultaneously; the box door and the interface port door are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the box and interface port doors. A mechanical elevator lowers the two doors, with the cassette riding on top, into the canopy covered space. A manipulator picks up the cassette and places it onto the cassette port/elevator of the equipment. After processing, the reverse operation takes place.
The SMIF system has been proved effective by experiments using prototype SMIF components both inside and outside a clean room. The SMIF system provides at least a ten-fold improvement over the conventional handling of open cassettes inside the clean room.
Conventional SMIF pods have the potential to create particles due to physical scraping of latch mechanisms against latch surfaces. Although few particles are released during each latching operation, over the course of hundreds of processing steps for a wafer, the number of particles created each time the SMIF pod is latched and/or unlatched may build to a significant number.
In SMIF pods utilizing conventional latching systems, scraping occurs when a latch member and a latch surface, one or both of which have a sloped surface, move from a disengaged to a fully engaged orientation. The sealing surfaces on the box and the box door are designed to seal upon application of pressure which forces these sealing surfaces together, and thus the latch system must (a) prevent movement of the box door with respect to the box and (b) create a clamping force which forces the sealing surfaces together. The clamping force is generated by sliding the latch surface along a sloped latch member or vice versa.
Latch mechanisms on conventional SMIF pods are usually provided on the box so that the latch mechanism may alternately be used to clamp the box door to the box or to clamp the box to the port plate of a processing station. To provide this dual function, it is necessary to have notches or openings in the perimeter of the bottom of the box so that the latch mechanisms could move to engage either the box door or the port plate. These openings provide access points through which contaminants may enter the "clean" environment.
In conventional SMIF pods, the latch mechanism supports the edges of the box door creating the possibility that the box door will deflect (bow or bend) under the weight of the wafers resting on the box door in addition to the sealing forces, potentially creating leaks in the seal between the box and the box door. Further, conventional latch mechanisms for SMIF pods may be subject to tampering which allows unauthorized access to the contents of the container.